1. Field of the Invention
This application relates to detection of chemical traces of substances such as explosives or narcotics and, more particularly, to the non-contact collection of particles of the substances from a surface that is moving with relative motion to a trace sample collection system.
2. Description of Related Art
There exist a wide variety of instruments that are capable of detecting and identifying particles of narcotics or explosives once the sample of particles is transported to the instrument and subsequently vaporized. Examples include, but are not limited to, ion mobility spectrometers, mass spectrometers, gas chromatographs, surface acoustic wave sensors, cantilever beam sensors, and electron capture detectors. Similarly, there are several ways commonly employed to transport said particles to the instrument, some of which are incorporated within the instrument and some requiring an operator to perform the transfer. Examples include, but are not limited to, mechanically transporting a collected sample to the instrument, vacuum collection of vapor or particles, and vortex vacuum sampling.
In the above examples, the particles begin by being attached to a surface by weak chemical bonds, van der Waals forces, mechanical attachment in a fibrous structure or porosity, electrostatic attraction, or entrainment in a sticky material, such as grease. For narcotics and explosives particles, the surface adhesion forces can be relatively strong, making the particles difficult to remove by simple, low momentum transfer methods, such as blowing a puff of air. Removal of such strongly adhered particles by blowing air is usually successful only for the largest, heaviest particles that present the greatest surface area to the blowing air. In general, blowing air does not readily remove particles of explosives or narcotics from rigid surfaces, only from flexible surfaces, such as cloth, where the fluttering motion of the material provides the momentum to mechanically dislodge the particles, or from unstable surfaces, such as cardboard, where the substrate material can flake off together with the target particle. Even with cloth, the blowing air stream usually requires a very high velocity flow to have any effect and then only for the largest particles, so the process is very inefficient. Surfaces subject to blowing air during normal usage, such as the sides of a vehicle, are particularly difficult for obtaining a trace chemical sample simply by employing an air jet.
The distance between the target surface and the blowing air jet is also relevant. Air jets from nozzles are known to diverge and slow in velocity with distance traversed due to interaction with the surrounding atmosphere, making them lose efficiency for particle removal with increasing standoff distance. A nozzle that employs an aerosol that includes pressurized gas and solid particles in order to enhance target particle release is similarly affected, and the aerosol particles slow rapidly with standoff distance.
In some cases, the process of taking a sample begins with an operator or a machine physically wiping an absorbent, often textured substance, such as chemical filter paper, onto the surface to be tested. Particles of the chemical of interest may then be transferred and concentrated on or in the surface texture of the absorber by the mechanical action of the wiping. This intermediate absorber is then brought to the vicinity of the detection instrument to make a measurement. The wiping method generally works reliably and efficiently but can be costly, because the media usually has to be replaced often.
There are many applications in which it is desirable to avoid having to manually wipe a surface to obtain trace particles. These include sampling without an operator, large area sampling, remote sampling, robotic sampling, and situations in which the frequent replacement of wiping materials is not acceptable. Examples of applications desirable for non-contact sample acquisition include the examination of people, packages, baggage, mail, objects on a manufacturing production line, and vehicles. However, the targets of each of these applications are often in motion, for example, items on a moving belt, walking persons, and moving vehicles. While it is possible to require that these targets stop moving during the sensing process, it would be preferable to examine these targets while they are in motion relative to the trace sample collection system. It would also be preferable for a trace sample collection system to operate with similar efficiency whether it is the target object that is moving with a static trace sample collection system or the trace sample collection system is moving with a static target object. In some cases, both target object and trace sample collection system may preferably be in motion. Examples in which the sample acquisition system is in motion with a static target object include a robot scanning along the side of a suspect vehicle, and a human explosives detection portal in which the sample acquisition system scans along the side of a person.
Accordingly, it would be desirable to provide a trace sample collection system that allows for the efficient collection of a target particle sample while the target surface and the trace sample collection system are in relative motion.